Embodiments of the invention may comprise a single high-gain antenna, or an array of such antennas, fed from a radio frequency source for radiating electromagnetic energy--or connected to a receiver for reception of such energy. In order to embrace both alternatives, in this specification, the term "driven element" will be used for that element of the antenna which, for transmission, would be driven by a signal source to provide radiation but, for reception, would be connected to the receiver.
Microstrip antennas usually are printed on a dielectric substrate, backed by a ground plane, and radiate in the direction normal to the ground plane, with little or no radiation along the ground plane. They offer numerous advantages, including low weight, low profile and ease of fabrication using printed circuit technology. The latter becomes increasingly important at high microwave frequencies, where the signal wavelength is short and maintaining fabrication tolerances is difficult to achieve by other techniques. In addition, microstrip antennas can easily be integrated with electronics, which makes them ideal candidates for applications using integrated electronics. Achieving higher gains is also relatively easy using microstrip antenna technology. Several radiating, driven elements can be printed on the same substrate, to form an array, and fed using conventional microstrip line feed networks. Beam scanning is also possible by placing phase shifts between the array elements. However, the scan range is limited for microstrip arrays.
Because each microstrip antenna radiates normal to its ground plane, the array gain decreases rapidly for angles near the ground plane. In other words, hitherto, microstrip antennas and their arrays have not been capable of high gain radiation parallel to the plane of their arrays. This is a major limitation of microstrip antennas.
The introduction of mobile satellite communications systems, such as MSAT, has resulted in a need for low-profile directional microstrip antenna configurations which can conveniently be conformed to, for example, an aircraft wing or land vehicle roof. U.S. Pat. No. 5,220,335 (Huang) discloses such an antenna having a driven element, a reflector and two directors, all of which are microstrip patch elements and coplanar. According to Huang, (Col. 5, line 16) one embodiment of his invention tilts the antenna beam about 40 degrees from the usual normal direction i.e. perpendicular to the plane of the patches, while a second embodiment provides only 30 degrees of tilting (Col. 5, line 34). Hence, true end-fire radiation is not achieved.
A similar microstrip antenna, but without the reflector element, is disclosed in U.S. Pat. No. 4,370,657 (Kaloi). Thus, Kaloi's antenna has a microstrip patch driven element and two coplanar parasitic director elements. It too does not achieve true end-fire radiation.
An object of the present invention is to overcome the limitation of these known antennas and provide a low-profile microstrip antenna capable of higher gain in the plane of the antenna.